Laser plasma tube having internal gas path

ABSTRACT

Several embodiments of a gas laser plasma tube are described having one or more internal gas return paths doubled back upon themselves within the structure which defines the gaseous discharge bore, to increase their length so that electrical gaseous discharge in the same is inhibited.

BACKGROUND OF THE INVENTION

The present invention relates to lasers and, more particularly, to a gaslaser plasma tube having a gas return path within the structure definingthe discharge cavity, which return path is of a design inhibitinggaseous discharge along such path even in relatively high poweredlasers.

Ion gas lasers produce coherent radiation from an electrical gaseousdischarge. Such gaseous discharge takes place within a plasma tubehaving structure defining a discharge cavity between anode and cathodeassemblies. Such structure typically is a cylinder of a high temperaturematerial, such as ceramic, positioned between the anode and cathode, andthe discharge cavity is an axial bore extending through such cylinder.

The gaseous discharge causes gas to flow through the discharge cavityfrom one end to the other. It is therefore common practice to includestructure of one sort or another defining a gas return path extendingbetween the ends of the discharge cavity to allow the pressures at suchends to be equalized. Traditionally, the gas return path in most lasershas been defined by structure, such as glass tubing, provided as anexternal appendage to the plasma tube. Such external arrangements,though, are relatively fragile and are not uncommonly broken. To avoidsuch fragility, lasers are now made incorporating the gas return pathdirectly into the structure of the plasma tube which defines thedischarge cavity. In those lasers in which the structure providing thedischarge cavity is a cylinder of a high temperature material, the gasreturn path typically is provided by one or more bores which extendthrough the cylinder parallel to the discharge cavity. Care must betaken with such a structure, however, to assure that the gaseousdischarge is confined to the discharge cavity and is not also initiatedalong the gas return path bores. Various approaches have been used inthe past to so restrict the gaseous discharge. U.S. Pat. No. 3,624,543describes one approach in which the gas return path bores generally havea smaller transverse cross-sectional area than the discharge cavity.Since the gaseous discharge will tend to follow the path of leastresistance between the anode and cathode assemblies, suchcross-sectional relationship will cause the discharge to take placepreferentially in the discharge cavity. This approach is not entirelysuitable, however, for relatively high powered lasers, i.e., lasershaving an output power rating of 2 watts or more. That is, the samereduction in cross-section of the gas return paths used to inhibitgaseous discharge also inhibits gas return flow therethrough. Besideshigher powered lasers having a greater anode-cathode potentialdifferential making discharge in an internal gas return path morelikely, they typically have a higher rate of gas flow through thedischarge cavity and it is difficult to obtain sufficient return gasflow for smooth operation. Another approach to preventing discharge ininternal gas return path bores has been to insert restrictions, such asmesh or thin discs, in the return paths to inhibit gaseous discharge.Again, such restrictions also may tend to inhibit gas flow and reducethe effectiveness of the path to return gas as intended.

SUMMARY OF THE INVENTION

The present invention provides a plasma tube for a gas laser having agas return path within the structure which also defines the dischargecavity, which return path effectively inhibits gaseous discharge throughthe same while not appreciably inhibiting gas flow therethrough. In itsmost basic aspects, the gas return path is made to double back uponitself within the structure which defines the discharge cavity. In thisconnection, the phrases "double back upon itself" and "doubling backupon itself" as used to describe the gas return path mean that the pathincludes directional components in opposition to one another, and notthat the path itself is doubled in length.

The gaseous discharge will take place preferentially along the path ofleast resistance between the anode and cathode, and it has been foundthat the increased length of the gas return path associated with thedoubling back will inhibit such discharge from taking place therein,without requiring that the return path have a component which isexterior of the structure defining the discharge cavity. Thus, gaseousdischarge will take place preferentially in the discharge cavity withoutthe gas return path having to be defined by appendages or other specialmeans outside of the structure which defines the discharge cavity. Infact, depending upon the amount by which the gas return path is madelonger than the discharge cavity by such doubling back, the transversecross-sectional area of the gas return path can be made as large andeven larger than that of the optical cavity. The result is that ratherthan restrict the flow of gas therethrough, the gas return path has ageometry which enhances gas conductance. In this connection, whileadding length to the gas return path may have a tendency to slow theresponse of the path to differential pressure changes at the oppositeends of the optical cavity, with the invention no slower response timehas been noted.

Most desirably, the discharge cavity defines a generally straight-linepath through the tube for the electrical gaseous discharge. Thisenhances the likelihood that the gaseous discharge will take place inthe discharge cavity rather than along the gas return path. That is, theline-of-sight between the anode and cathode assemblies provided by thedischarge cavity will encourage discharge therealong, whereas thenon-linearity of the gas return path will inhibit the same. Also mostdesirably the doubling back of the gas return path is provided byincluding one or more bends therein of generally 180° so that the pathwill have adjacent, generally parallel sections. This is simplyaccomplished by providing all or part of the cylinder containing thedischarge cavity with at least three separate bores defining the gasreturn path and means communicating opposite ends of a first one of suchbores with second and third ones of such bores, respectively.

The invention includes other features and advantages which will bedescribed in connection with the following description of severalpreferred embodiments.

BRIEF DESCRIPTION OF THE DRAWING

With reference to the accompanying three sheets of drawing:

FIG. 1 is an elevation view of an ion gas laser incorporating apreferred embodiment of the invention;

FIG. 2 is an enlarged, partly-sectioned view illustrating the plasmatube and electrode assemblies of the preferred embodiment of theinvention illustrated in FIG. 1;

FIG. 3 is an exploded, isometric view of the plasma tube of thepreferred embodiment illustrated in FIG. 1;

FIGS. 4 and 6 are schematic sectional views illustrating alternatepreferred embodiments of the invention; and

FIG. 5 is an isometric view of another preferred embodiment of a plasmatube of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A preferred embodiment of a gas ion laser incorporating the presentinvention is illustrated in FIGS. 1 through 3 of the drawings. Laser 11includes an elongated plasma tube 12 enclosed within a cooling waterjacket 13. In the type of laser construction to which the presentinvention relates, the plasma tube 12 consists essentially only of thatstructure which defines the gaseous discharge cavity. To this end,plasma tube 12 has an axial bore 14 extending lengthwise thereof whichencloses a lasable medium, i.e., quantum resonant particles, such as anionized noble gas, at a pressure of about 1 torr. Plasma tube 12 is acylinder of a high temperature material, e.g., BeO ceramic, made up ofthree cylindrical segments 16, 17 and 18 which are axially aligned andplaced end-to-end. The discharge cavity 14, i.e., the volume within theplasma tube within which substantially all of the discharge is confined,is defined at least in part by aligned, separate axial bores in thethree segments. In the preferred embodiment being described, the axialbore 14 provides essentially the full volume within which the dischargeis confined and is therefore structurally synonomous with the dischargecavity.

As is conventional, means are included for initiating gaseous dischargewithin discharge cavity 14. That is, cathode and anode assemblies 19 and20, respectively, are provided at opposite ends of the plasma tube. Apower supply, schematically represented at 21 in FIG. 1, is alsoconventionally provided as part of such means for applying the requisitedifferential voltages to the cathode and anode for the desired gaseousdischarge. The discharge will generate optical radiation, i.e., visible,infrared or ultraviolet radiation, in the discharge cavity 14 of thetube for propagation along the axis 22 (FIG. 1) of the laser, whichradiation will pass through windows at the opposite ends of the plasmatube and be reflected by optical reflector assemblies or mirrors 23 backand forth through the discharge cavity 14 a sufficient number of timesto sustain laser oscillation. Transmission of optical radiation throughone of the reflectors as represented at 24 constitutes the coherentradiation output of the laser.

Laser 11 further includes resonator structure which maintains thereflectors or mirrors 23 at its opposite ends at predetermined relativeorientations and a set distance apart. More particularly, it includes apair of rectangular reference plates 26 and 27 positioned adjacentopposite ends of the laser optical cavity. Plates 26 and 27 are rigidlyheld in position relative to one another by three rods 28 (two of whichare shown) connected between three of the four corners of the respectivereference plates. The rods 28 are selected to have low thermal expansionin the direction of the laser optical axis over the ambient temperaturerange to which they are expected to be subjected during operation of thelaser. The rods 28 and, hence, the remainder of the laser are supportedby uprights 29 extending upwardly from a base 31.

Reference plates 26 and 27 are used to provide a reference base fromwhich the optical reflectors 23 of the laser are mounted on oppositeends of the optical cavity. In this connection, the reflector assembliesinclude rectangular mounting plates 32 and 33 positioned parallel andadjacent the respective reference plates 26 and 27. The mounting platesare connected to the reference plates by the tuning bolt-leaf springseparator arrangement described in U.S. Pat. No. 3,864,029 issued Feb.4, 1975, the disclosure of which is hereby incorporated by reference.

As mentioned previously, the electrical discharge tends to cause gas toflow through the discharge cavity, with the consequence thatdifferential pressures tend to be created at the anode and cathodeassemblies. In order to equalize such pressure, it is the practice toprovide a gas return path between the ends of the discharge cavity,separate and apart from such cavity. It has not been practical, however,to utilize internal gas return paths, i.e., paths which extend throughthe cylinder of high temperature material which defines the dischargecavity, in higher powered lasers. The higher potential differences whichmust be applied between the anode and cathodes to obtain the higheroutput power sometimes cause the gaseous discharge to follow gas pathsother than the one defined by the discharge cavity. Moreover, the rateof change of pressure between the ends of the discharge cavity is oftengreater than can be handled by conventional designs of internal gaspaths.

The present invention provides a simple internal gas return path designwhich effectively inhibits discharge therein without affecting thecapability of such path to equalize the gas pressure at the oppositeends of the gas discharge cavity. Such gas return path is similar toprior designs of internal gas return paths in the sense that it isprovided by bores extending through the segments 16, 17 and 18. However,in keeping with the invention, the gas return path is made substantiallylonger than the discharge cavity by being doubled back upon itself.Because of this doubling back, the transverse cross-sectional area ofthe cavity can be made sufficiently great to accommodate the amount ofgas flow required in higher powered lasers without causing discharge toalso occur in the gas return path. Moreover, the gas return path isnon-linear between the ends of the optical cavity, whereas the dischargecavity defines, as is usual, a straight-line path between the anode andcathode. The non-linearity in the gas return path will not significantlyaffect gas flow therethrough but it will inhibit the formation of adischarge arc between the differing potentials which cause the same.

Most simply and effectively, the gas return path is defined by at leastthree separate bores which extend through at least part of the plasmatube cylinder 12, one of such bores being communicated with the othertwo at its opposite ends, respectively. That is, the center segment 17has three separate gas return path bores defining a doubled-back gasreturn path through such segment having adjacent, generally parallelsections. In this connection, means are provided to communicate theopposite ends of one of such bores, bore 37, with the other two bores,bores 36 and 38, for serial flow therethrough. Such means includes, inthis preferred embodiment, at each end of the segment 17, channels 39and 41, respectively, in those ends of segments 16 and 18 adjacent thesegment 17. Channel 39 is positioned to communicate one end of the bore37 with a first end of the bore 38, whereas channel 41 is positioned tocommunicate the other end of the bore 37 with the corresponding end ofbore 36. The gas return path bore 42 in segment 18 is aligned with theother end of bore 38.

Gas return flow along the resulting path will be described assuming flowfrom the cathode end of the plasma tube to the anode end as representedby the arrows 44. Gas flowing from the cathode end will enter thesection of the gas return path defined by the bore 42 in the segment 16,flow through bore 36 in segment 17 and be doubled-back by channel 41 forflow through bore 37 also in segment 17. Channel 39 will double backinto bore 38 gas exiting the path section provided by bore 37, for flowagain through segment 17. The flow from the path section provided bybore 38 will pass into the path section defined by the bore 43 insegment 18, and thence to the anode end of the plasma tube.

It will be recognized from the above that the internal gas return pathprovided by the respective bores in the segments 16 and 17 and 18includes two 180° bends, i.e., the two bends provided by the channels 39and 41. If the segments 16, 17 and 18 are of equal length, it will beseen that the doubling back in the segment 17 causes the gas return pathto have essentially twice the length of the discharge cavity, permittingfor good gas conductance the transverse cross-sectional area of the gasreturn path to be generally at least as great for substantially its fulllength as the transverse cross-sectional area of the discharge cavity.

It will be recognized that the path sections are defined in each of thesegments by straight-line bores. This facilitates construction of thegas return path. The channels which provide the bends in the path aresimply formed at the exposed ends of segments. It should be noted thatas used herein the term "bore" is not meant to connote any particularmanner in which the void or voids to which it refers is constructed, northat such void necessarily has a circular cross-section.

It will be apparent from the above that various arrangements canincorporate the internal gas return path design of the presentinvention. For example, a number of segments different from three couldbe provided. FIG. 4 illustrates a five, end-to-end segment plasma tubeincorporating the present invention. Two of such segments, segments 46and 47 are each provided with three gas return path bores. Such segmentsare separted by the other three segments 48, 49 and 51, and thecommunication channels are provided in the ends of such other segmentswhich are adjacent segments 46 and 47 having the gas return path bores.A multiple of segments greater than three are often desirable in higherpowered lasers which require a greater length of plasma tube cylinder.

While in both embodiments described to this point, the segments havingthree bores abut segments which only have one bore, it is not necessarythat they be so arranged. For example, in the embodiment of FIG. 4, thesegment 49 can be removed from between the segments 46 and 47 so thatthe three sections of one communicate directly with the three pathsections of the other. It will be noted, though, that such directcommunication of such multiple path section segments with one anothereliminates some doubling back. In the embodiment of FIG. 4, the removalof the segment 49 would eliminate the two 180° bends provided by itbetween the segments 46 and 47.

FIG. 5 illustrates another preferred embodiment of the inventionincorporated into a plasma tube cylinder which is essentially a singleintegral piece. With reference to such figure, the illustrated singlepiece plasma tube 53 has in it for its full length three bores definingthree separate sections of the gas return path. The means communicatingsuch bores are channels 54 and 56 respectively provided in the surfacesof caps 57 and 58 at the opposite ends of the cylinder 53. The channels54 and 56 are similar to the communication channels utilized in thepreviously described embodiments in that they communicate opposite endsof a first one of the gas return path bores within the cylinder 53 withthe second and third bores therewithin, respectively. Flow of return gasinto and out of cylinder 53 is provided by bores 59 and 61 extendingrespectively through caps 57 and 58.

The gas return path of the embodiment of FIG. 5 passes three separatetimes through the cylinder 53, with the result that such path is threetimes as long as the discharge cavity.

The invention is also applicable to arrangements in which more than oneinternal gas return path is provided through the plasma tube. That is,more than one separate gas return path can be provided through the tubeto increase the gas conductance, any number of which can have theincreased length and non-linear features of the present invention. Theembodiment illustrated in FIG. 6 has a single piece plasma tube cylinder62 having two separate gas return paths, each one of which is defined bythree separate bores extending through the cylinder. In the schematicshowing in FIG. 6, the separate paths are shown on the opposite sides ofthe discharge cavity 63. It will be recognized, however, that in anactual construction the bores through the cylinder providing the pathsections preferably are equally spaced apart, to maximize the amount ofstructural material therebetween. The upper path 64 is defined by threebores communicating with one another in the manner discussed in theearlier embodiment. The second gas return path 66 is similarly definedby three bores below the discharge cavity 63. With such an arrangement,the transverse cross-sectional area of the individual gas return pathscan be made smaller than the transverse cross-sectional area of thedischarge cavity to further inhibit the formation of a gaseous dischargein the former. However, for best conductance, it is preferred that thetotal transverse cross-sectional area of the plurality of gas returnpaths be at least as great as the transverse cross-sectional area of thedischarge cavity. While in this embodiment the channels providingcommunication between the bore sections are defined by caps 67 atopposite ends of the cylinder 62, a plurality of gas return pathsincorporating the present invention can be provided in cylindricalsegments of the type incorporated in the embodiments of FIGS. 1-3 andFIG. 4.

Although the invention has been described in connection with preferredembodiments thereof, it will be appreciated by those skilled in the artthat various changes can be made without departing from its spirit. Itis therefore intended that the coverage afforded applicant be limitedonly by the spirit of the invention as set forth in the claims and theirequivalents.

We claim:
 1. A plasma tube for a gas laser comprising structure havingat least a portion of a generally straight-lined path discharge cavityextending lengthwise therethrough along which electrical gaseousdischarge takes place for the generation of coherent energy, saidstructure also defining a gas return path extending therethrough fromone end thereof to the other for return flow of gas from one end of saiddischarge cavity to the other, said gas return path including at leastthree sections within said structure having directional componentsparallel to the discharge cavity path, which sections are communicatedwith one another for serial flow therethrough in opposite directions toincrease the length of the gas return path and thereby cause gaseousdischarge to take place preferentially in said discharge cavity.
 2. Agas laser plasma tube according to claim 1 wherein the transversecross-sectional area of said gas return path is at least as great forgenerally the full length of said path as the transverse cross-sectionalarea of said portion of said discharge cavity within said structure. 3.A gas laser plasma tube according to claim 1 wherein said structuredefines a bend in said gas return path of generally 180°, whereby saidpath is doubled back to have adjacent generally parallel sections.
 4. Agas laser plasma tube according to claim 1 wherein said structure has aplurality of said gas return paths extending therethrough from one endthereof to the other.
 5. A gas laser plasma tube according to claim 4wherein the total transverse cross-sectional area of said plurality ofgas return paths is at least as great as the transverse cross-sectionalarea of said portion of said discharge cavity within said structure. 6.A gas laser plasma tube according to claim 1 wherein said portion ofsaid discharge cavity is defined by a bore extending through a cylinderof a high temperature material providing said structure, and said gasreturn path is provided by a separate bore or bores extending throughsaid cylinder.
 7. A gas laser plasma tube according to claim 6 whereinsaid gas return path is defined by three separate bores extendingthrough said cylinder; and means are provided to communicate at a firstend of said cylinder a first one of said gas return path bores with asecond one of such bores, and means are provided to communicate at theopposite end of said cylinder said first one of said gas return pathbores with the third one of said bores.
 8. A gas laser comprising aplasma tube having a lasable medium, structure defining a generallystraight-lined path discharge cavity extending therethrough forelectrical gaseous discharge for the generation of optical energy, andmeans for initiating gaseous discharge within said discharge cavity,said structure also having a gas return path extending therethrough fromone end thereof to the other for return flow of gas from one end of saiddischarge cavity to the other, said gas return path including at leastthree sections within said structure having directional componentsparallel to the discharge cavity path, which sections are communicatedwith one another for serial return gas flow therethrough in oppositedirections to increase the length of the gas return path and therebycause gaseous discharge to take place perferentially in said dischargecavity.
 9. A gas laser according to claim 8 wherein said structuredefines a plurality of said doubled-back gas return paths extending fromone end thereof to the other, the total transverse cross-sectional areaof said plurality of gas return paths being at least as great as thetransverse cross-sectional area of said discharge cavity.
 10. A gaslaser according to claim 8 wherein said structure defines a bend in saidgas return path of generally 180°, whereby said path is doubled back tohave adjacent, generally parallel sections.
 11. A gas laser according toclaim 8 wherein said structure includes a cylinder of a high temperaturematerial having a bore extending therethrough defining at least aportion of said discharge cavity, and said gas return path is providedby a separate bore or bores extending through said cylinder.
 12. A gaslaser according to claim 11 wherein said gas return path is defined byat least three separate bores in said cylinder; and means are providedto communicate a first end of a first one of said gas return path boreswith a second one of such bores, and means are provided to communicatethe opposite end of said first gas return path bore with the third oneof said bores.
 13. A gas laser according to claim 12 wherein saidrespective means to communicate said first and opposite ends of saidfirst one of said gas return path bores with said second and third onesof said bores are caps at the ends of said cylinder defining channels atsaid bores ends providing said communication.
 14. A gas laser accordingto claim 11 wherein said cylinder is comprised of an end-to-endplurality of cylindrical segments of said high temperature material;said gas return path is defined by at least three separate boresextending through at least one of said segments; and means are providedto communicate at a first end of said segment a first one of said gasreturn path bores with a second one of such bores, and means areprovided to communicate at the opposite end of said segment said firstgas return path bore with the third one of said bores.
 15. A gas laseraccording to claim 14 wherein said means to communicate at a first endof said segment a first one of said gas return path bores with a secondone of such bores includes structure defining a channel in the end ofanother segment adjacent said first end of said segment, extendingbetween the ends of said first and second gas return path bores in saidone segment.
 16. A gas laser according to claim 14 wherein there are atleast three end-to-end segments of high temperature material definingsaid cylinder, one of which is provided with said three gas return pathbores.
 17. A gas laser according to claim 16 wherein said segment havingsaid three gas return path bores is the center one of said threeend-to-end segments, and said respective means to communicate a firstone of said gas return path bores with said second and third ones ofsaid bores includes structure defining a channel in each of the ends ofthe other segments adjacent said center segment extending between saidfirst gas return path bore and said second and third bores thereof,respectively.
 18. A gas laser according to claim 16 wherein there are atleast five end-to-end segments of high temperature material definingsaid cylinder, at least two segments of which are separated by othersegments and are provided with three gas return path bores, means areprovided to communicate at a first end of each of said two segments afirst one of the gas return path bores therein with a second one of suchbores, and means are provided to communicate at the opposite end of eachof said two segments the first return path bore therein with the thirdone of said bores, said respective means incuding a channel in each endof another segment adjacent said respective two segments extendingbetween said first gas return path bore and said second and third boresthereof, respectively.